1. Field of the Invention
The present invention relates to a differential apparatus for a vehicle, and more particularly to a differential apparatus comprising an orthogonal axis gear that transmits the power of a front-rear extending input shaft to left and right output shafts.
2. Description of the Related Art
An apparatus disclosed in Japanese Unexamined Patent Application Publication 2004-278762 is known as a conventional differential apparatus comprising a hypoid gear, which is used widely as an orthogonal axis gear for a vehicle.
As shown in FIGS. 3 and 4 (FIGS. 1 and 2 of Japanese Unexamined Patent Application Publication 2004-278762), in this differential apparatus (11), the rotation direction of a front-rear extending input shaft (30) for transmitting engine power transmitted from a center differential apparatus (10) is shifted ninety degrees by a drive pinion gear (41) and a ring gear (35), and then the engine power is transmitted to a differential case (36), whereupon the engine power is transmitted to left and right output shafts (45), (46) by a differential gear mechanism constituted by pinion gears (39), (40) that rotate integrally with the differential case (36) and side gears (37), (38) that mesh with the pinion gears (39), (40).
With a hypoid gear, greater gear strength and a larger speed reduction ratio than those of a zero-offset bevel gear can be secured by offsetting the drive pinion gear, but as the offset increases, the transmission efficiency during a low load period deteriorates. Furthermore, the gear reaction force also increases in accordance with the offset of the hypoid gear, and therefore a tapered roller bearing, which requires a pre-load that can resist a powerful meshing reaction force, must be provided as the bearing of the input shaft. However, a tapered roller bearing is more expensive than a ball bearing and exhibits greater rolling resistance. Further, the tooth surface load and sliding velocity of the hypoid gear are large, and therefore sufficiently viscous lubricating oil must be used. However, the viscous resistance of such lubricating oil is great. Hence, a hypoid gear may cause deterioration of the fuel economy, thereby affecting the fuel economy of a vertically installed engine vehicle or a four-wheel drive vehicle comprising the hypoid gear.
Accordingly, the use of a small offset hypoid gear or a zero offset bevel gear in place of a large offset hypoid gear has been considered.
An apparatus disclosed in Japanese Unexamined Patent Application Publication S64-6548 is known as a differential apparatus employing a bevel gear.
As shown in FIG. 5 (FIG. 1 of Japanese Unexamined Patent Application Publication S64-6548), in this differential apparatus, the rotation direction of the power of a front-rear extending input shaft (1) is shifted ninety degrees by a small bevel gear (2) and a large bevel gear (3) forming the bevel gear, and then the power is transmitted to a housing (4). The power is then input into a sun gear (12) of a planetary gear reduction mechanism (5) that rotates integrally with the housing (4), a ring gear (14) of the planetary gear reduction mechanism (5) being fixed, and is then output from a planetary carrier (15) of the planetary gear reduction mechanism (5) to a differential box (6) that rotates integrally with the planetary carrier (15) inside the housing (4). Finally, the power is transmitted to left and right output shafts (9), (9) by a differential pinion (7) that rotates integrally with the differential box (6) and a differential side gear (8) that meshes with the differential pinion (7).
However, with the conventional differential apparatus disclosed in Japanese Unexamined Patent Application Publication 2004-278762, the differential case (36), which transmits power from the ring gear (35) to the differential gear mechanism and houses the pinion gears (39), (40) and side gears (37), (38), is disposed on the axial periphery of the left and right output shafts (45), (46). Likewise in the conventional differential apparatus disclosed in Japanese Unexamined Patent Application Publication S64-6548, the housing (4), which transmits power from the large bevel gear (3) to the planetary gear reduction mechanism (5) and houses the differential gears (7), (8), is disposed on the axial periphery of the left and right output shafts (9), (9). Hence, in both the former and the latter, the axial periphery of the left and right output shafts increases in diameter, leading to an increase in the size of the differential case.